1. Technical Field
This invention relates generally to communication systems and, more particularly, to integrated time references within a device for use either as a clock reference or calibration master within portable electronics including radio frequency (RF) wireless devices operating in wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wired communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wireless application protocol (WAP), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel of the other parties (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and exchange information over that channel. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched (PSTN) telephone network, via the Internet, and/or via some other wire lined or wireless network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.) to participate in wireless communications. As is known, the receiver receives RF signals, removes the RF carrier frequency from the RF signals via one or more intermediate frequency stages, and demodulates the signals in accordance with a particular wireless communication standard to recapture the transmitted data. The transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard and adds an RF carrier to the modulated data in one or more intermediate frequency stages to produce the RF signals.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier (LNA), zero or more intermediate frequency (IF) stages, a filtering stage, and a data recovery stage in many designs. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The down converters mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency signal. As used herein, the term “low IF” refers to both baseband and low intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
As the demand for enhanced performance (e.g., reduced interference and/or noise, improved quality of service, compliance with multiple standards, increased broadband applications, etc.), smaller sizes, lower power consumption, and reduced cost continue to be asserted, wireless communication device engineers are faced with a very difficult design challenge to develop a wireless communication device that satisfies requirements that sometimes appear to be mutually exclusive.
Integrated time references are used for many purposes, including synchronization of internal operations, synchronization with buses and external networks, among other applications. For example, for a device that communicates over an external synchronized bus, it is important that the device has an internal time reference that it can use to detect and respond to the signals on the bus. Generally, synchronized buses require that all operations happen at specified instants in time. Thus, a device must not only be able to read the synchronized signals being received on the bus, but must also be able to transmit at specified instants in a synchronized manner.
Crystal oscillators have long been used to provide very accurate time keeping functions as a result of their steady and predictable response to physical or electrical stimuli. Integrated circuits, however, by their very nature, cannot incorporate an internal crystal oscillator. Accurate internal time keeping is needed, for example, by analog-to-digital converter (ADC) circuits. ADCs are complex analog-to-digital converters that are often used to digitize analog wave forms, for example, voice wave forms, as a part of converting a voice signal to a digital signal that may be manipulated, stored or transmitted over a wireless medium. Other circuits that require accurate time keeping are the frequency generation circuits, such as phase-locked loops, so that ingoing and outgoing communication signals may readily be exchanged with other devices.
More specifically, the conversion of the voice signal from analog to digital will be most accurate and most reproducible if the sampling occurs at precise and constant measures of time. A transmitter must be able to accurately drive a signal on a synchronized bus. Thus, for these and many other reasons, a need exists not only for internal time sources that may be used as a reference signal, but also for accuracy. At the same time, however, there is the ever increasing need or desire to reduce power consumption in electronic circuits, especially for portable devices, to conserve battery life. Because of the desire to reduce power consumption, especially for portable devices, it is customary for a transceiver to operate in a sleep mode during periods of inactivity and in a normal mode while processing data. The sleep mode is provided to avoid wasting power when data is not being transmitted, received or processed. When a device is to end a sleep mode, it must periodically wake-up and attempt to establish a connection with nearby devices. The timing for waking up is required to be reasonably accurate, nonetheless. Additionally, when the device wakes up, it must be able to lock with a specified clock or with a received RF signal in order to accurately process data. Thus, most current designs include clock systems that consume sufficient power to provide accurate time keeping. Known designs that reduce power consumption during a sleep mode often fail to provide the desired accuracy. What is needed, therefore, is a system that maintains adequate clock timing while reducing power consumption.